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ROSWELL, N.M. — The 12-year-old boy who opened fire on a crowd of students in a New Mexico middle school gym had planned the attack and warned some classmates to stay away moments before the gunfire rang out, investigators said Wednesday.

State Police Chief Pete Kassetas said it appeared the victims in Tuesday's shooting at Berrendo Middle School in Roswell — an 11-year-old boy and 13-year-old girl — were chosen randomly.

The shotgun used by the boy came from his family's home, and he had three rounds of ammunition, Kassetas said at a news conference.